Thermoplastic compounds, unlike wood, metal, or glass, do not rot, rust or shatter. For that reason, the world in the past seventy years has seen a revolution in material science arising from the combination of thermoplastic resin and one or more functional additives to provide specific properties to the resin.
During processing, polymers are heated to reach a molten state for mixing with other ingredients and then forming into the shape of the final article. However, once the final polymeric article is made, the melting and even potential burning of the article when exposed to excessive heat or open flame can be detrimental to property and persons. Some polymers, such as polyvinyl chlorides, are inherently more able to withstanding exposure to heat and/or an open flame without melting or burning. On the other hand several commonly used polymers such as polyolefins, polyesters, and polycarbonates are more vulnerable to melting or burning.
Therefore, the plastic industry has focused on making polymeric materials having improved flame retardancy. Flame retardant additives, drip suppressants, mineral fillers and char formers are used as functional additives to help thermoplastic compounds retard the effects of heat or flame from causing a material to melt or even burn. Many early flame retardants relied on halogen-containing compounds, but more recently several non-halogenated fame retardants have been developed. Non-halogenated flame retardants have become popular, because they minimize the release of halogenated chemicals if the plastic article would begin to degrade, melt or burn.
Polyphosphonates are highly desirable as non-halogenated flame retardants that can be included as an additive to improve fire retardancy of thermoplastic and elastomeric compounds. However, compared to other types of non-halogenated flame retardants, polyphosphonates have a high affinity for capturing moisture, increasing the risk of hydrolysis and degradation when these compounds are exposed to heat and humidity. Degradation of polyphosphonates leads to lower molecular weight, and consequently a loss of mechanical properties, such as tensile strength, impact resistance and tensile modulus. In addition, the fire resistance of the polyphosphonate becomes diminished due to the lower molecular weight, which increases the melt flow properties, and thereby causes the material to be more likely to drip when exposed to a flame or heat.
Polyphosphonate compositions containing functional additives to improve stability when exposed to heat and moisture are disclosed in U.S. Pat. No. 7,666,932 (Freitag). However, these additives are limited to sterically hindered phenolic antioxidants, hydrolytically stable organophosphites, organophosphites, antioxidants, sterically hindered lactone antioxidants and combinations thereof. Moreover, U.S. Pat. No. 7,666,932 explicitly teaches that epoxy oligomers provide no improvement to the stability of polyphosphonate.